The present invention relates to current limiting fuses used in electrical power distribution systems and particularly to improving the thermal withstand capability of the filament wound epoxy tube body of the fuse during long term low current interrupting conditions.
A current-limiting fuse consists of a fusible element wound on a core surrounded by clean sand or silica, contained inside an insulating tube. This tube can be made from a glass, ceramic or glass filament wound epoxy body depending upon the fuse design. The body made from filament wound epoxy tubing is economical and hence is popularly used. The fusible element melts on over-currents and creates an electric arc which burns inside the sand. This heat energy is absorbed by the melting sand or silica and the arc is quenched.
The fuse body must be capable of maintaining its mechanical and electrical characteristics at higher temperatures. The fuse while carrying its normal load current can be exposed to higher ambient temperatures up to 40.degree. C. (as per ANSI/IEEE C 37.1940-1981). In addition, for general purpose current limiting fuses, the fuse body temperature could rise during melting on long term low currents. In addition, the arcing inside the sand adds to the heat input. The arcing times are longer at lower available currents.
In a full range power force described in U.S. Pat. No. 4,638,283, in addition to the heating of the tube due to arcing, the chemical charges which are located along the length of the element fire and add heat to the system. Thus the energy dumped is higher than the conventional general purpose fuses and the fuse body should be capable of withstanding these thermal stresses.
The current rating of a current-limiting fust as defined by ANSI/IEEE C 37. 40-1981, is dependent on the maximum temperature withstand capability of the fuse body. This limits the maximum current rating in a particular body size of a fuse The fuse design engineer cannot use all the available temperature withstand capability of the fuse body to increase its current rating because of the thermal withstand requirements during interruption of long term low currents described above.
The present market trend in current-limiting fuses is toward low cost, higher current rated fuses in smaller body sizes. This requires the use of filament wound epoxy tubing rather than glass or ceramic tubes The ceramic or glass tubing does not burn during the excess heat period, described above, while the low cost filament wound epoxy tubes tend to burn when used in designs where the arcing occurs close to the fuse body. Thus the distance between the fusible element and the body determines the current rating or the body size of a fuse. In the chemically augmented fuses, this is more critical because of the chemical charges located on the elements.
It is therefore the object of the present invention to shield the fuse body of a current-limiting fuse from the hot gases of an electric arc or from the firing of the chemical charge and permit the fuse element to be positioned closer to the fuse body to permit a smaller fuse.
It is a further object of the invention to shield and protect a fuse body formed of a low cost inexpensive material normally prone to burning when it is exposed to direct flow of hot gases.
These and other objects of the present invention are obtained by providing a ceramic shield which can withstand a very high temperature gradient to protect the inner surface of the filament wound epoxy body of a low cost fuse tube.
Other objects, features and advantages of the present invention will be more fully apparent from the following detailed description of the preferred embodiment, the appended claims and the accompanying drawings.